This invention relates to a method for polishing an optical lens.
The process of preparing optical or ophthalmic lenses begins with an unfinished or semi-finished glass or plastic lens blank. Typically a semi-finished lens blank has a finished polished front surface and an unfinished back surface. By grinding away material from the back and/or front surface of the lens blank the required corrective prescription is generated.
Thereafter the surface having had the corrective prescription imparted thereto is polished and the peripheral edge of the thus processed lens blank is provided with a final desired contour thereby establishing a finished optical or ophthalmic lens.
The grinding process may create surface roughness on the surface of the lens, which tends to undesirably scatter light passing to or from the lens. To reduce this surface roughness, the lens is polished to obtain a smoother surface.
The lens blank can be either a plastic or a glass lens blank. Blanks used for eyeglasses typically are made by injection molding or casting a thermosetting polymer such as di ethylene glycol bis(allyl carbonate) (CR-39) or polycarbonate.
Most automated cutting machines have a cutter that is held stationary while rotating the lens and moving it along two axes with respect to the cutter. If the lens requires a curvature in addition to simple spherical and/or cylindrical cuts, the lens can be ground while tilted to produce an offset optical center (i.e. an induced prism).
After the lens is cut, it is polished. Polishing method may include polishing the surface of the optical lens with the aid of a polishing tool for example a polishing pad.
Some optical lenses after the grinding step may have so called sharp edges. That is edges that may damage the polishing tool during the polishing step.
A problem linked to the sharp edges is that during the polishing step the polishing tools are prematurely damaged for example the polishing pad is worn or the foam is torn off by cutting.
In the case of glass with sharp edges the lifetime of the polishing tool may be divided by 5 or 10 compared to the case of glass with no sharp edges.